staging: rtl8192u: Remove casts on the return values of list_entry

Remove casts return values of on all occurances of list_entry.
Casts on the return values of list_entry are useless.
list_entry is a macro and already casts its return value to the type
mentioned in its second argument.

Signed-off-by: Bhanusree Pola <bhanusreemahesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>